Messages in this thread Patch in this message | | | Date | Tue, 14 Sep 2010 03:07:19 -0400 | From | Mathieu Desnoyers <> | Subject | [RFC PATCH] sched: START_NICE feature (temporarily niced forks) |
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This patch tweaks the nice value of both the parent and the child after a fork to a higher nice value, but this is only applied to their first slice after the fork. The goal of this scheme is that their respective vruntime will increment faster in the first slice after the fork, so a workload doing many forks (e.g. make -j10) will have a limited impact on latency-sensitive workloads.
This is an alternative to START_DEBIT which does not have the downside of moving newly forked threads to the end of the runqueue.
Latency benchmark:
* wakeup-latency.c (SIGEV_THREAD) with make -j10 on UP 2.0GHz
Kernel used: mainline 2.6.35.2 with smaller min_granularity and check_preempt vruntime vs runtime comparison patches applied.
- START_DEBIT (vanilla setting)
maximum latency: 26409.0 µs average latency: 6762.1 µs missed timer events: 0
- NO_START_DEBIT, NO_START_NICE
maximum latency: 10001.8 µs average latency: 1618.7 µs missed timer events: 0
- START_NICE
maximum latency: 9873.9 µs average latency: 901.2 µs missed timer events: 0
On the Xorg interactivity aspect, I notice a major improvement with START_NICE compared to the two other settings. I just came up with a very simple repeatable low-tech test that takes into account both input and video update responsiveness:
Start make -j10 in a gnome-terminal In another gnome-terminal, start pressing the space bar, holding it. Use the cursor speed (my cursor is a full rectangle) as latency indicator. With low latency, its speed should be constant, no stopping and no sudden acceleration.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> --- include/linux/sched.h | 2 ++ kernel/sched.c | 16 ++++++++++++++-- kernel/sched_fair.c | 30 +++++++++++++++++++++++++++++- kernel/sched_features.h | 6 ++++++ 4 files changed, 51 insertions(+), 3 deletions(-)
Index: linux-2.6-lttng.git/kernel/sched_features.h =================================================================== --- linux-2.6-lttng.git.orig/kernel/sched_features.h +++ linux-2.6-lttng.git/kernel/sched_features.h @@ -12,6 +12,12 @@ SCHED_FEAT(GENTLE_FAIR_SLEEPERS, 1) SCHED_FEAT(START_DEBIT, 1) /* + * After a fork, ensure both the parent and the child get niced for their + * following slice. + */ +SCHED_FEAT(START_NICE, 0) + +/* * Should wakeups try to preempt running tasks. */ SCHED_FEAT(WAKEUP_PREEMPT, 1) Index: linux-2.6-lttng.git/include/linux/sched.h =================================================================== --- linux-2.6-lttng.git.orig/include/linux/sched.h +++ linux-2.6-lttng.git/include/linux/sched.h @@ -1132,6 +1132,8 @@ struct sched_entity { u64 prev_sum_exec_runtime; u64 nr_migrations; + u64 fork_nice_timeout; + unsigned int fork_nice_penality; #ifdef CONFIG_SCHEDSTATS struct sched_statistics statistics; Index: linux-2.6-lttng.git/kernel/sched.c =================================================================== --- linux-2.6-lttng.git.orig/kernel/sched.c +++ linux-2.6-lttng.git/kernel/sched.c @@ -1829,6 +1829,8 @@ static void dec_nr_running(struct rq *rq static void set_load_weight(struct task_struct *p) { + unsigned int prio; + if (task_has_rt_policy(p)) { p->se.load.weight = 0; p->se.load.inv_weight = WMULT_CONST; @@ -1844,8 +1846,10 @@ static void set_load_weight(struct task_ return; } - p->se.load.weight = prio_to_weight[p->static_prio - MAX_RT_PRIO]; - p->se.load.inv_weight = prio_to_wmult[p->static_prio - MAX_RT_PRIO]; + prio = min_t(unsigned int, MAX_PRIO, + p->static_prio + p->se.fork_nice_penality); + p->se.load.weight = prio_to_weight[prio - MAX_RT_PRIO]; + p->se.load.inv_weight = prio_to_wmult[prio - MAX_RT_PRIO]; } static void enqueue_task(struct rq *rq, struct task_struct *p, int flags) @@ -2421,6 +2425,8 @@ static void __sched_fork(struct task_str p->se.sum_exec_runtime = 0; p->se.prev_sum_exec_runtime = 0; p->se.nr_migrations = 0; + p->se.fork_nice_timeout = 0; + p->se.fork_nice_penality = 0; #ifdef CONFIG_SCHEDSTATS memset(&p->se.statistics, 0, sizeof(p->se.statistics)); @@ -2482,7 +2488,13 @@ void sched_fork(struct task_struct *p, i if (p->sched_class->task_fork) p->sched_class->task_fork(p); + if (sched_feat(START_NICE)) { + struct rq *rq = this_rq(); + p->sched_class->put_prev_task(rq, current); + set_load_weight(p); + p->sched_class->set_curr_task(rq); + } /* * The child is not yet in the pid-hash so no cgroup attach races, * and the cgroup is pinned to this child due to cgroup_fork() Index: linux-2.6-lttng.git/kernel/sched_fair.c =================================================================== --- linux-2.6-lttng.git.orig/kernel/sched_fair.c +++ linux-2.6-lttng.git/kernel/sched_fair.c @@ -513,6 +513,16 @@ __update_curr(struct cfs_rq *cfs_rq, str delta_exec_weighted = calc_delta_fair(delta_exec, curr); curr->vruntime += delta_exec_weighted; + if (curr->fork_nice_penality && curr->fork_nice_timeout != -1UL) { + curr->fork_nice_timeout -= delta_exec; + /* + * We cannot update load here while task is enqueued, so + * perform the update lazily at the end of the current (or next) + * dequeue. + */ + if ((s64)curr->fork_nice_timeout < 0) + curr->fork_nice_timeout = -1UL; + } update_min_vruntime(cfs_rq); } @@ -832,6 +842,16 @@ dequeue_entity(struct cfs_rq *cfs_rq, st */ if (!(flags & DEQUEUE_SLEEP)) se->vruntime -= cfs_rq->min_vruntime; + + /* + * We can only set the weight back to its normal value when the task is + * dequeued. + */ + if (se->fork_nice_timeout == -1UL) { + se->fork_nice_penality = 0; + set_load_weight(task_of(se)); + se->fork_nice_timeout = 0; + } } /* @@ -3544,8 +3564,16 @@ static void task_fork_fair(struct task_s update_curr(cfs_rq); - if (curr) + if (curr) { se->vruntime = curr->vruntime; + if (sched_feat(START_NICE)) { + curr->fork_nice_timeout += sched_slice(cfs_rq, curr); + curr->fork_nice_penality += 3; /* about 50% lighter */ + se->fork_nice_timeout = curr->fork_nice_timeout; + se->fork_nice_penality = curr->fork_nice_penality; + set_load_weight(p); + } + } place_entity(cfs_rq, se, 1); if (sysctl_sched_child_runs_first && curr && entity_before(curr, se)) { -- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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